ESP-WROOM-32 to microcontroller firmware

[xennex]

I wish to use an ESP-WROOM-32 PCB module in another board which has a microcontroller. I can do this now using the UART interface and AT command set but are concerned about maximum throughput speed and the use of the text based AT protocol.
1. With standard firmware, what is the maximum throughput of the UART/AT protocol? For example if I want to download a 100MByte file using WiFi, assuming the WiFi connection is fast, at 115200 baud it will take 2.5 hours, which is not acceptable for my project. Is there a way to change the baud rate? If it involves reflashing with custom firmware, then we may as well move to the next question:
2. How hard is it to implement I2C (slave) or SPI (slave) on the ESP32? I'm guessing I would need to download firmware source and toolchain, make changes and reflash the ESP32. What are the maximum speeds of each alternative interface? My preference is SPI as then I don't need to bother with the software stack on the master end and I find it more reliable, but I see that I2C is far more common.
3. This leads to the next item, which is that I would prefer to use a protocol other than text based AT commands. I programmed modem communications in the early 90s and are shocked that someone would pick the Hayes command set in this day and age. Or saddened, or both shocked and saddened. A binary protocol which gives you packets sizes and checksums in responses rather than polling for OK text strings, and well as power on garbage, would be a much more reliable solution. So, are there any pre-made firmwares which use other protocols?
Thanks for any help.

[xennex]

Found one answer:
1. Use AT+UART_CUR to raise the baud rate (or UART_DEF if you are brave). In testing I've found that lowering the baud rate works, but raising does not work, but I'm hoping that my firmware version will allow higher speeds.